The present invention relates generally to code division multiple access (CDMA) communication networks, and more particularly, to interference reduction and cancellation in mobile CDMA networks.
A third generation (3G) code division multiple access (CDMA) can concurrently supports multiple services and multiple symbol rates on multiple channels. For example, services can include data services at relatively high symbol rates, and voice services at relatively low symbol rates. It is very difficult to find a simple solution that supports variable symbol rates for the various services, in real time, particularly, when the channels are confined to a fixed frequency and fixed bandwidth, i.e., a fixed xe2x80x9cchipxe2x80x9d rate.
In a CDMA network, the symbols representing data and voice signals can be orthogonally modulated using quadrature variable length Walsh codes to distinguish the respective channels and minimize cross-correlation. Error correction is typically carried out by convolution coding using variable encoding rates for the respective channels. Combining the CDMA with a spread-spectrum technique can increase network capacity. This makes each orthogonally coded control and traffic channel of the CDMA network practical for broadcast and mobile digital cellular networks (DCN).
A variable length Walsh code can be denoted as WNM, where N is the Nth row of the Walsh metrics, and M is the length of a specific Walsh code W. For a channel with a given bandwidth, e.g., 1.25 MHz, and a fixed spreading/modulation method, e.g., quadrature phase shift keying (QPSK), higher symbol rates are modulated with shorter length Walsh codes and lower symbol rates are modulated with longer Walsh codes. This code assignment ensures that the symbols on the various channels remain orthogonal.
For a desired frame error rate (FER), the signal power must vary with the symbol rate. Usually, high symbol rate data services require a lower FER than low symbol rate voice services. Consequently, on a channel providing data services, the symbols are transmitted at a relatively higher signal power than on channels providing voice services. As a result, high symbol rate channels can interfere with low symbol rate channels, especially in a mobile communications network, such as a cellular phone network where a base station concurrently transmits symbols for data and voice services on multiple channels. Depending on the relative locations of the mobile receivers, the interference can become so serious that low power signals cannot be recovered.
Therefore, it is desired to reduce interference in a wireless communications network that simultaneously supports asymmetric services such as Internet data access and voice communications in real time, particularly when the receivers in the network are mobile.
The invention provides reliable communication channels in a CDMA network. The invention is particularly suited for mobile networks where symbols are concurrently transmitted over multiple fixed frequency and fixed bandwidth channels at variable symbol rates and signal powers. More particularly, the invention provides reliable low signal power voice services in the presence of high signal power data services that otherwise would interfere to meet associated application needs. More specifically, the invention can provide a mobile user good voice communications while experiencing strong interference from high data rate users.
The invention method and apparatus reduces interference adaptively in a CDMA receiver. A baseband signal is demodulated to determine target symbols. The baseband signal is also demodulated to estimate interfering symbols. The interfering symbols are modulated to generate a cancellation signal, and the cancellation signal is subtracted from the base band signal to reduce interference.